Junior ADU Definition - What does Junior Accessory Dwelling Unit mean?

Junior Accessory Dwelling Units (JADUs) are small living units (less than 500 square feet in size) like a studio apartment or efficiency, that exist within a single family residence.

In other words, JADUs are a powerful tool if you’re thinking about converting a room of your house into a potential rental unit for extra income, or to house a family member.

This article includes:

2 key concepts about JADUs
The legal definition of JADUs in the state of California
How the definition can vary in your city or county
JADU Pros and Cons
When should you build a JADU vs other types of housing
How much does building a JADU cost
Alternatives to Junior Accessory Dwelling Units

As always, this is not legal advice - the information on this website is presented as general information. You should speak to a licensed professional in your jurisdiction and work with local agencies on your project!

Two important JADU concepts if you’re short on time

If you only take two things away from this article, I hope it’s these two concepts:

Make the right decision: Junior ADUs aren’t the best solution for every homeowner, but they’re perfect for some homeowners.
Do your research: cities and counties are interpreting JADU code VERY differently and the interpretations are evolving quickly, so it’s important to know what you can do on your specific property at this specific moment in time. This article is a starting point, but your local agency is the ultimate enforcer of what can be built and approved. You can get more comprehensive help with your project by filling out this form.

What is the legal definition of Junior ADUs in the state code?

Junior ADUs have their own definition in California’s state building code that differentiate them from other Accessory Dwelling Units in some important ways.

“Junior accessory dwelling unit” means a unit that is no more than 500 square feet in size and contained entirely within a single-family residence. A junior accessory dwelling unit may include separate sanitation facilities, or may share sanitation facilities with the existing structure.
Section 65852.22 of the Government Code

In plain English, here are some important concepts about Junior ADUs [edited 10/30/2020 to reflect new HCD guidance]:

They’re “Dwelling units” that you can rent out legally
They have to be less than 500 square feet
They’re contained within a single-family residence (SFR)
You can build both an ADU and a Junior ADU on the same property
[Begin edit:] Some jurisdictions say[end edit] a detached ADU is an SFR, so a JADU may be permitted inside a proposed detached ADU ("The accessory dwelling unit may be combined with a junior accessory dwelling unit")
JADUs can share a bathroom with the residence
JADUs need their own kitchenette (see “efficiency kitchen” requirements)
JADUs need their own entrance/exit (see “passageway” requirements)
JADUs have an owner occupancy requirement
JADUs have fewer utility separation requirements than conventional ADUs
~~You can add up to 150 square feet to accommodate JADU ingress/egress~~ [edit 10/30/2020: I’ve struck this line out. While an argument can still be made for a bump out based on a literal reading of the state code, the HCD has issued guidance in September 2020 that this clause is not meant to be applied to JADU conversions]

Here’s a useful definition from Los Angeles County:

“Junior ADU (JADU) means a unit that is no more than 500 sq. ft. in size and contained entirely within an SFR [single-family residence], including an attached garage. A JADU must have a separate entrance from the SFR. A JADU may share a bathroom with the main house. A JADU must have an efficiency kitchen at a minimum. Either the JADU or the SFR must be owner occupied. A property with an SFR is permitted one JADU and one detached ADU in total.”
Los Angeles County Department of Regional Planning Interim Summary, June 30 2020

[edit: one day after publishing, this content was removed from the LA County website. Here is a link to the LA County Interim Summary pdf hosted by how-to-adu.com]

That said, this is the definition for LA County - it isn’t the same in every part of California.

There are very important variations in how Junior ADU regulations are being interpreted and enforced around the state…

How the definition of a JADU can vary by county and by city

The biggest and most common difference I’ve seen from one jurisdiction to another is whether a homeowner can convert non-living space like a garage into a Junior Accessory Dwelling Unit, or whether they are limited to using existing livable square footage like a bed room or study.

[Edit 10/30/2020:] The California Department of Housing and Community Development (HCD) has issued new guidance in September 2020 clarifying some of these discrepancies. You can read more about that update and download the new handbook including the guidance. [/edit]

There are many other variations that I have seen or heard of:

Whether the JADU has to be in the primary residence or can it be built within a proposed detached conventional ADU
Whether there is an owner occupancy requirement on the primary residence or not [edit 10/30/2020: the HCD update has clarified that there is an owner occupancy requirement on either the JADU or primary residence]
Whether the JADU counts as an additional “EDU” (Equivalent Dwelling Unit) for annual water and sanitation charges [edit 10/30/2020: the HCD update has clarified that JADUs should not be counted toward connection charges, but can be counted toward annual fees]
Whether the homeowner can build outside the footprint of the residence for ingress/egress [edit 10/30/2020: the HCD update has clarified that the 150 square foot bump out for ingress/egress provided by the state code only apply to ADUs and do NOT apply to JADUs unless your local jurisdiction decides to allow that]

Some of these are pretty critical differences in interpretation.

For example, if you have a 1200 square foot home with a 264 square foot garage, converting your garage into a JADU and increasing the square footage of your house by over 20% is very exciting.

How can there be so many conflicting definitions?

While there is a state-wide code that defines Junior Accessory Dwelling Units, there are two ways that local agencies (cities, counties, water companies, etc) can create variations.

They can pass ordinances further restricting construction, and they can interpret and enforce the state code in different ways.

Each city and county in California may pass ordinances that affect JADUs in their jurisdictions, which can change what you can and cannot do with a JADU. For example, the city may pass laws protecting historic buildings in a historic district that would prevent you from modifying the facade to create an egress for a JADU.

And furthermore, even in the absence of local ordinances or guidelines, the local planning department and inspectors may interpret and enforce the state code differently. The debate about whether garages are included in single-family residences or not is a quintessential example of this difference in interpretation.

How to learn the definition in your city or county

To figure out how your planning department treats Junior Accessory Dwelling Units, you should figure out your jurisdiction and google its junior accessory dwelling unit policy. (Here’s a link to a detailed tutorial on how to do that.)

If you’re in the jurisdiction of Sonoma County, search for “Junior Accessory Dwelling Unit Sonoma County”
http://sonomacounty.ca.gov/PRMD/Regulations/Housing/Housing-Types/Accessory-Units-and-Junior-Units/
Check the date of the ordinance - in this case, Sonoma County expressly says “As of January 1, 2020, the County's ADU ordinance no longer applies.”
So until Sonoma County passes new ordinances, their old ordinance is defunct, and they have to use state code
But they specifically say “A Junior Accessory Dwelling Unit (JADU)…utilizes an existing bedroom” which is a hint that they’re interpreting the state code to exclude garage conversions
So if you want to pursue a JADU garage conversion in Sonoma County, you should contact the planning department and seek a clarification about whether that’s allowed. Don’t go too far or spend too much time and money before getting that sorted out.

Okay, the example gets even better…

If you happen to live in Santa Rosa, which is in Sonoma County, then you get a totally different result.

Santa Rosa passed an ordinance in March 2020 that explicitly says “The junior accessory dwelling unit shall be contained entirely within the existing walls of an existing or proposed single-family dwelling, which includes the walls of an attached garage“

So this can vary tremendously even within the same county!

If reading city and county ordinances, and writing strongly worded letters is NOT your idea of a good time, that’s one of the things we do for you at How To ADU. The first hour or so is free because we can usually help you out with existing articles and resources. Just fill out this form to get started.

    Pros
    Cons
  
    Can share sanitation
    Owner occupancy requirements
  
    Cheaper fees and charges
    Size limited <500 sq ft
  
    Fewer utility separation requirements
    Unless you're converting garage, basement, etc., it's not very different from renting a room out – and potentially more expensive
  
    Can be built in addition to an ADU
  
    May add to square footage of a house*

Pros	Cons
Can share sanitation	Owner occupancy requirements
Cheaper fees and charges	Size limited <500 sq ft
Fewer utility separation requirements	Unless you're converting garage, basement, etc., it's not very different from renting a room out – and potentially more expensive
Can be built in addition to an ADU
May add to square footage of a house*

JADUs vs ADUs - Pros and Cons

It’s probably obvious by now that Junior ADUs have some strengths, but they also come with limitations.

The table to the right has what I see as the biggest pros and cons of “Junior Accessory Dwelling Units” compared to conventional “Accessory Dwelling Units”.

Some lists also point out that JADUs are relatively cheap and avoid impact fees, but this is also true of a small ADU so I’ve left it out of the list.

The advantages

Cheap - Sharing sanitation and utilities makes it very easy and cheap to create a Junior ADU. In some extreme cases, you can add a door to the exterior, a small kitchenette, and call it a day.

Depending on your jurisdiction, you may also save money on permitting because some areas say “Junior units are not separate dwelling units and don’t require building or fire code upgrades…“ (Sonoma County for example) If you only have to pay planning and building permit fees, that can save you thousands.

It also might reduce capacity and connection charges for water and sewer because JADUs that are within the existing primary residence aren’t supposed to be considered new residential use.

“Triplex” - Another incredible strength is that you can build a Junior ADU in addition to a conventional detached ADU. What’s more, you can build it in the detached ADU!

Add square footage - The last thing on the advantages list has a big asterisk next to it because it varies by jurisdiction. Some cities and counties allow the homeowner to convert non-living space like a garage into livable square footage.

This is especially important in jurisdictions like Los Angeles where many homes have built up to the maximum square footage the city will allow. Garage conversion via JADU is a way to add square footage to the house despite these limitations (when the local jurisdiction allows it).

The disadvantages

Owner occupancy - The owner occupancy is a big drawback if you want to develop a rental property, or if you want to keep that option open for the future. But it doesn’t affect most of the homeowners who consult with How To ADU, because they plan on living in the primary residence anyway.

Smaller size - The size limit isn’t overly burdensome either since 500 square feet is quite a bit of space (your average 2 car garage is right around 576 square feet).

Permit costs - This is technically a con versus Non-ADUs. The cost of permitting a Junior ADU, while it may be lower than a conventional ADU, can still be relatively high depending on the city/county. And some homeowners may be just as happy renting out a room of their house to boarders. Renting a room out has a lot of the same benefits of developing a Junior ADU or conventional ADU and it might be less costly.

When should you build a Junior ADU?

Once you know how the city or county interprets Junior Accessory Dwelling Unit regulations, you can make an educated decision.

Here are some basic questions to ask yourself:

Do you mind the owner occupancy requirement? If you do, then a JADU is probably not a good option for you.
Are you building a detached ADU that could include a JADU? If so, then this is something worth looking into, especially if you’re trying to develop rental income. If it’s for your family, you probably don’t need the extra JADU, but it might give you more options in the future.
Can you add to the square footage of your home by converting a garage, basement or other structure into a JADU? If your local city or county planning department allows it, this is a great way to do it because you don’t need to use up space for an extra bathroom.
Are you trying to bring a non-conforming studio or apartment up to code? If you want to bring some non-conforming work up to code, JADUs are sometimes a cost effective way to do it.

Here’s a video about Marian who built a Junior Accessory Dwelling Unit in Sebastopol for less than $10,000.

How much does it cost to build a JADU?

Junior Accessory Dwelling Units have a huge range in cost because they can be almost anything from exceptionally affordable conversion projects, ranging to brand new construction within a proposed conventional Accessory Dwelling Unit.

There aren’t a tremendous number of Junior Accessory Dwelling Units in the market so it’s tough to provide an average cost.

So please take this next section with a big grain of salt. It’s not rigorous; it’s based on the numbers homeowners have shared with me and other ADU experts.

The most affordable JADU I can imagine

If you’re converting a bedroom that already has an exterior door, you might just be adding a kitchenette and paying for permits and inspection. You could get out for less than $20,000 and have a full-fledged JADU.

It’s a bit of a dream scenario. And it also assumes you’re in a city or county with low building and planning fees.

The more expensive, most typical price range of JADUs

At the expensive end, a JADU conversion designed to add square footage to your house might be quite expensive.

It can even be more expensive than a detached construction because you’re working hard to preserve the existing residence’s structure while doing serious work (eg. retrofitting foundations, framing, etc.)

Just to put a number on it, I’ve heard of small garage conversions costing above $150,000.

Realistic expectations for your potential JADU project

You probably want a realistic ballpark figure to know if you should keep thinking about JADUs or not.

If you’re building a detached ADU and want to include a JADU within the proposed building, you’re going to be looking at very similar cost per square foot. I’d say the incremental costs are negligible in most cases and you should look into this option.
If you’re trying to add square footage to your house by converting a garage/basement/etc., I want to set your expectations that this is often a very expensive project. Assume the same costs as new detached construction per square foot.
If you’re trying to bring a non-conforming studio or apartment up to code, this can vary wildly. In the dream scenario, the studio is already up to code and you’re just paying for permits and inspections. In the nightmare scenario, there are big problems with the non-conforming space and you are facing the same costs as the above bullet.
If you’ve got a room that’s already finished as living area and you only need to do marginal work like adding an exterior door or a kitchenette, then you might get away with a very affordable JADU.

To get a clearer picture of your costs, we need two things:

We need to visit your residence and really evaluate the potential for a JADU
We need more JADUs to be built so we can get more data!

Here’s a more rigorous post about the costs of building conventional Accessory Dwelling Units to show how I’d like to thin about JADUs once we have more information.

Alternatives to JADUs

Junior ADUs aren’t the only way to rent out an existing room to a tenant.

Renting a room out of your house

If you have a permitted, finished room and want to lease it out, you might be able to do that without a JADU.

Things that can get in the way are if your mortgage agreement forbids letting out a room or if you’re in an HOA or some other covenant that prohibits long term rentals. That said, JADUs might give you a way around these limitations on letting out a room in your primary residence.

On the other hand, if your room was never permitted, then there’s a great reason to get it permitted as a JADU. Renting out non-conforming rooms is illegal and potentially dangerous. Paying for the permitting and inspection process to bring the room up to code gives you the legal right to rent it out.

A Conventional “Accessory Dwelling Unit”

You can also get a normal Accessory Dwelling Unit (ADU).

If you don’t plan on building TWO ADUs, and if you want to put an independent bathroom into the conversion, you could get it permitted as a conventional ADU and avoid the owner occupancy requirement.

We don’t know yet how much value the market will price in for JADUs vs conventional Accessory Dwelling Units, but I imagine the extra bathroom and the freedom from owner occupancy requirements will make ADUs more appealing than JADUs in the long run.